Frequency Planning
Understanding Frequency Planning
Frequency planning is one of the most critical and often underestimated steps in receiver design. A poor frequency plan leads to spur-related performance problems that cannot be fixed after the architecture is set.
Frequency Planning Steps
- Define the RF tuning range.
- Choose first IF: high enough for image rejection, low enough for practical filters.
- Determine LO range and injection side (high-side or low-side).
- Generate spur chart: plot m*fRF +/- n*fLO for m,n = 1 to 5.
- Verify no spurs fall in the IF passband across the entire tuning range.
- Add second IF if needed for channel selectivity.
- Verify image frequency rejection with available filter technology.
Frequently Asked Questions
What is frequency planning?
Systematic selection of all frequencies (RF, LO, IF) to avoid spurs, image responses, and internal interference. The spur chart (m*fRF +/- n*fLO) must show no in-band products across the tuning range.
Why is dual conversion used?
First conversion: high IF for easy image rejection (image is 2*IF away). Second conversion: low IF for narrow channel filters. This gives both good image rejection and good selectivity. Standard for high-performance receivers.
What IF frequencies are common?
First IF: 70 MHz (legacy), 140 MHz, 1-2 GHz (modern). Second IF: 10.7 MHz (FM), 21.4 MHz, 70 MHz. Higher first IF simplifies image rejection. Lower second IF enables narrow crystal or SAW filters.